The Trench Decision: Conduit or Direct Burial

Every outbuilding feeder eventually becomes a hole in the ground, and the hole forces a choice: UF-B cable buried directly, or THWN conductors in PVC conduit. The choice barely changes the voltage drop math — but it completely changes what the drop math is worth later.

The two systems

Direct-burial UF-B is cheapest on day one: trench to the required depth, bed it, bury it. PVC conduit with THWN costs more in parts and labor but is shallower in some cases under local rules, mechanically protected, and — decisively — re-pullable.

Why conduit wins the long game

Voltage drop sizing is a bet on future loads. The garage that needs a 30 A circuit today wants a 60 A feeder when the EV arrives and 100 A when the workshop gets serious. With UF-B, each upgrade is a new trench. With 1¼″–2″ conduit, it is a Saturday and a pull rope: yank the old conductors, pull the next size. Oversizing the conduit one trade size costs a few dollars per stick; oversizing the trench twice costs a weekend and a lawn. Check your planned pull in the fill calculator — and then check the size you might want in ten years.

Sizing notes for buried runs

• Buried routes are longer than they look — measure the actual trench path with its bends and risers, not the site-plan diagonal, before running the solver.
• Earth is a kind thermal environment; the standard 75 °C ampacity columns serve normal residential burial well, with conductor temperature rarely the governing constraint at these loads.
• Aluminum shines underground: at feeder sizes the cost gap is huge, and the terminations live in dry boxes at each end. See copper vs aluminum.